Electrical conductive film

ABSTRACT

An electrically conductive film for use in printed circuits and the like. The film has a thermosetting binder which contains metal powder in flake form and metal powder in dendrite form dispersed therein. The metal powder in flake form is distributed more densely in the layer at the surface remote from the base on which the film is placed than in the layer at the surface of the film which is against the base.

Waited States Kojima et al.

[451 Get. 23, 1973 ELECTRICAL CONDUCTIVE FILM Inventors: Kunio Kojima; Kazuyuki Shimada;

Mitsuo Wada, all of Osaka, Japan Matsushita Electric Industrial C0,, Ltd., Osaka, Japan Filed: Jan. 31, 1972 Appl. No.: 222,008

Assignee:

Foreign Application Priority Data Feb. 3, 1971 Japan 46/4399 Feb. 9, 1971 Japan 46/5754 US. Cl. 161/168, 117/38, 117/160 R,

161/162, 161/D1G. 7 Int. Cl. B32b 5/16, B32b 5/14 Field of Search 161/162, 166, 168,

l61/D1G. 7; 117/38, 160 R [56] References Cited UNITED STATES PATENTS 3,287,202 11/1966 Petriello 161/162 3,386,001 5/1968 Slosberg et a1. 161/162 3,708,387 l/1973 Turner et a1 161/168 Primary Examiner-William J. Van Balen Att0meyE. F. Wenderoth et a1.

[57] ABSTRACT An electrically conductive film for use in printed circuits and the like. The film has a thermosetting binder which contains metal powder in flake form and metal powder in dendrite form dispersed therein. The metal powder in flake form is distributed more densely in the layer at the surface remote from the base on which the film is placed than in the layer at the surface of the film which is against the base.

8 Claims, 1 Drawing Figure 1 ELECTRICAL CONDUCTIVE FILM This invention relates to an electrically conductive film having metal powder in a flake form and metal powder in a dendrite formv dispersed in an organic binding material.

There have been known various electrically conductive films having finely divided metal powder dispersed in an organic binding material. For example, silver paint having silver powder dispersed in a vehicle including organic binding material is commercially available and forms an electrically conductive film upon being cured. In the silver paint, the higher the amount of organic binding material, the higher the adhesion of the electrically conductive film to a base plate but the poorer the surface conductivity of the film. On the other hand, the higher the amount of silver powder results in the higher the surface conductivity of the electrically conductive film but the poorer the adhesion to a base plate.

Therefore an object of this invention is to provide an electrically conductive film having metal powder dispersed in organic binding. material, said conductive film being characterizedby both a high surface conductivity and a high adhesion to a base plate.

This object is achieved by providing a conductive film according to the present'invention which has metal powder in a flake form and metal powder in a dendrite form dispersed in a thermosetting binder, said metal powder in a flake form being distributed more densely in the top layer of said'conductive film.

This object and other objects of this inventionwill be apparent upon consideration of the following detailed description taken together with the accompanying FIG- URE which is a cross-sectional view of aconductive film according to this invention, the section being greatly enlarged for easy understanding.

Referring to the FIGURE reference numeral 1 designates an insulating. base such as phenol, epoxy, meramin, urea or modifiedphenol resin-impregnated in paper or glassflber sheet. A conductive film adheres firmly to said insulating base 1 andhas flake metal particles 2 and dendrite metalparticles 3dispersed in thermosetting resin binder 4.

Said conductive film 5 has a specialconfiguration that metal particles in a flake form 2 are distributed more densely at a top layer of said conductive film 5 and dendrite metal particles3 are distributeduniformly in said conductive film 5.

The metal powder in a flake form is of about 1 to microns of diameter of flat surface and of about 0.01 to 0.1 microns of thickness.

The diameter of'the flat surface is measured-by electron microphotographs obtained by well known methods. The thickness is measured by electron microphotographs obtained by a conventional shadow cast method in which chromium metal or carbon is evaporated at an inclined angle to the sample powder and is deposited on the surface of the sample powder. According to thismethod, thesample powder leaves a portion having no chromium metal or carbon deposited thereon. The electron 'microphotographs'reveal a white portion. The thickness is calculated from the length of the white portion and the size of the angle, as described in Shadow Casting and Surface Replication of Chapter 13 in the bookVacuum Deposition of Thin Films" by L. Hollandrl960.

The metal powder in a flake form according to the present invention preferrably satisfies the following sedimentation test described below. A glass dish 100mm in diameter and 30mm deep is filled with butyl alcohol. 0.5gr. of metal powder is caused to fall on the surface of the butyl alcohol through a 325 mesh sieve. The metal powder is separated into two groups of particles; one floats on the surface of the butyl alcohol and the other sediments on the bottom of the glass dish. The floating powder is transfered to abeaker by decantation after being kept still for one minute and is dried. The weight ratio of floating powder to sedimented powder is hereafter designated the floating. ratio. A preferred metal powder in a flake form has the floating ratio more than 60%.

A use of metal powder in flake form makes it possible to form a conductive film having a higher surface conductivity at the top surface, i.e., the surface remote from the insulating base, than at the back surface, i.e. the surface against the base. Another advantage of the conductive film is that the back surface is rich in the thermosetting binder and adheres strongly to the base plate.

The metal powder in a dendrite form makes the conductive film mechanically stronger and thermally more suitable. Operable average particle sizes of the particles of said metal powder in dendrite form ranges from 10 to 15 microns.

The conductive film according to the present invention includes 10 to 60 wt. percent of metal particles in flake form and 40 to wt. percent of metal powder in dendrite form. The composite of said conductive film is 10 to 50 percent by volume of thermosetting binder and-50 to 90 percent by volume of the total of said metal powder in flake form andsaid metal powder in dendrite form.

The preferred metals for the particles in a flake form consist essentially of copper and silver. The preferred metals for the particles in dendrite form consist essentially of copper and iron. A combination of silver powder in'flake form and copper powder in dendrite form produces a conductive film having superior electrical conductivity, mechanical strengthand stability.

Said thermosetting binder consists essentially of a resin selected from the group consisting of phenol, xylene, urea, epoxy and modified phenol resin.

Said metal powder in a flake form can be prepared by any suitable and available crushing machine such as ball mill, stamp mill or vibration mill; The choice of the type of crushing machine must be decided by taking into account the brittleness of the starting materials. For example, a stamp mill produces a-more preferable copper powder in flake form when electrolytic copper powder is used as a starting material; Silver powder in a flake form is preferably prepared by using a ball mill, and electrolytic silver powder is used as a starting material. lt is important for preparation of metal particles in flake form that l to 20 weight percent of fatty acid such as stearic acid be added to the metal powder in the crushing machine. The fatty acid prevents the particles of metal powder in the crushing machine from oxidizing and adhering to each other. In addition, the use of fatty acid produces metal powder in flake form coated with a thin film of fatty acid which improves greatly the aforesaid floating ratio.

Said metal powder in a dendrite form consists of so called electrolytic metal powder which is obtained by electrolytic deposition well known in electrochemistry.

The metal powder in flake form and in dendrite form are dispersed in a liquid vehicle prepared by dissolving a thermosetting binder in a solvent for preparation of a conductive paste. The dispersion can be achieved by any suitable and available method such as a three roller mill or a ball mill. The consistency of said conductive paste is preferably adjusted in accordance with the method of application. Application methods which can be used are a spray method, dip method, brush method or screen stencil method. For example, the screen stencil method requires conductive paste with a viscosity of 500 to 2,000 poises.

The conductive paste needed to produce a given composition of resultant conductive film is applied to an insulating base. The conductive paste on the insulating base is cured at a temperature depending upon the thermosetting resin in the conductive paste and is formed into a conductive film according to the present invention. The thickness of conductive film may vary with the desired purpose and may range from to 50 microns.

The conductive film according to the present invention can easily be formed into a printed circuit having a desired pattern. The aforesaid conductive paste is applied in the desired circuit pattern to an insulating base such as an epoxy resin plate. The applied conductive paste is cured at a temperature depending upon the thermosetting binder included in said conductive paste. The application can be carried out by any suitable and available method well known in the art. The cured conductive paste having the desired circuit pattern forms a conductive film.

The following examples are exemplary embodiments of this invention and should not be construed as limitative.

EXAMPLE 1 l00gr. of silver powder is put in l00cc. of methyl alcohol at room temperature and stirred for 1 hour. The washed powder is dried at lC for 3 hours. The thus obtained silver powder is coated with a thin layer of stearic acid. The existence of a thin layer of stearic acid is proved by the following method. l0gr. of dried silver powder is cleaned with 50cc. of boiling alcohol in a reflux condenser for 1 hour. The boiled methyl alcohol has an acid value of 3.0 which proves the existence of stearic acid in the methyl alcohol.

The particles of silver powder are in flake form having a flat surface diameter of about 3 microns and a thickness of 0.03 microns as determined by the aforesaid electron microphotograph technique. The floating ratio of this silver powder is 75 wt. percent.

Copper powder in dendrite form is obtained by an electrolytic deposition method and is about 12 microns in diameter. The proportions on which the silver powder in flake form and the copper powder in dendrite form are mixed as listed in Table l.

The silver powder and the copper powder are dispersed well in a vehicle including 40 wt. percent of phenol resin as a thermosetting binder and 60 wt. percent of Carbitol by using a three roller mill. The thus obtained conductive paste has a volume proportion of 20 percent of phenol resin and percent of silver and copper powders after the Carbitol is evaporated. The conductive paste is applied to an epoxy resin plate by a I60 mesh stencil screen and is cured at l60C for 2 hours. The cured paste forms a conductive film having a thickness of 25 microns. The surface of said conductive film is predominantly silver powder and has a silvery appearance. The surface electrical resistivity and adhesive strength to the epoxy resin plate of the conductive films are as listed in Table 1.

TABLE 1 Mixing proportion Surface Adhesive (wt.%) Resistivity Strength Silver Copper (ohm/square) (Kg/cm) 10 0.5 7.5 35 65 0.3 6.0 40 60 0.2 4.5

EXAMPLE 2 Electrolytic silver powder having an average particle size of 15 microns is admixed with 5 wt. percent of stearic acid and pulverized in a stamp mill for 10 hours and subsequently in a ball mill for 5 days with the further addition of 5 wt. percent of stearic acid.

Iron powder in dendrite form is obtained by an electrolytic deposition method and has particles of about 10 microns in diameter. The powders are mixed in a mixing ratio of 35 wt. percent of silver powder in flake form to 65 wt. percent ofiron powder in dendrite form.

The silver powder and the iron powder are dispersed well in a vehicle including 40 wt. percent of phenol resin as a thermosetting resin and 60 wt. percent of Carbitol by using a three roll mill. The amount of vehicle relative to the total amount of silver powder and iron powder is adjusted so as to form a conductive film having various proportions of a total volume of silver powder and iron powder relative to the proportion of phenol resin as shown in Table 2 after the methyl Carbitol is evaporated off.

The conductive paste is applied to an epoxy resin plate by a mesh screen and is cured at 160C for 2 hours. The cured paste forms a conductive film having a thickness of 25 microns. The surface of said conductive is predominantly silver powder in a flake form and has a silvery appearance. The electrical surface resistivity and adhesive strength to the epoxy resin plate of the conductive films are as listed in Table 2.

TABLE 2 Volume Proportion Surface Adhesive Resistivity Strength (ohm/square) (Kg/cm) Phenol Silver resin and iron powders I0 90 0.4 5.4 30 70 0.6 7.5 50 50 0.8 |0.5

What is claimed is:

1. A conductive film comprising a thermosetting binder and metal powder in flake form and metal powder in dendrite form dispersed in said thermosetting binder, said metal powder in flake form being distributed more densely in the layer at one surface on said conductive film than at the other surface.

2. A conductive film as claimed in claim 1, wherein said metal powder in flake form has a floating ratio more than 60%.

3. A conductive film as claimed in claim 1, wherein said metal powder in flake form has flakes with a diameter across the flat surface of l to microns and a thickness of 0.01 to 0.1 microns and said metal powder in dendrite from has particles form '10 to microns in size.

4. A conductive film as claimed in claim 3, wherein said metal powder in flake form is present in an amount of 10 to 60 wt. percent of the total weight of the metal powder and said metal powder in dendrite form is present in an amount of 40 to 90 wt. percent of the total weight of the metal powder.

5. A conductive film as claimed in claim 4, wherein said conductive film has a composition consisting cssentially of 10 to 50% by volume of thermosetting binder and 50 to by volume of total metal-powder in flake form and dendrite form.

6. A conductive film as claimed in claim I, wherein the metal of said metal powder in flake form is one member selected from the group consisting of copper and silver.

7. A conductive film as claimed in claim 1, wherein the metal of said metal powder in dendrite form is one member selected from the group consisting of copper and iron.

8. A conductive film as claimed in claim 1, wherein the metals of said metal powder in flake form and metal powder in dendrite form are silver and copper, respectively. 

2. A conductive film as claimed in claim 1, wherein said metal powder in flake form has a floating ratio more than 60%.
 3. A conductive film as claimed in claim 1, wherein said metal powder in flake form has flakes with a diameter across the flat surface of 1 to 10 microns and a thickness of 0.01 to 0.1 microns and said metal powder in dendrite from has particles form 10 to 15 microns in size.
 4. A conductive film as claimed in claim 3, wherein said metal powder in flake form is present in an amount of 10 to 60 wt. percent of the total weight of the metal powder and said metal powder in dendrite form is present in an amount of 40 to 90 wt. percent of the total weight of the metal powder.
 5. A conductive film as claimed in claim 4, wherein said conductive film has a composition consisting essentially of 10 to 50% by volume of thermosetting binder and 50 to 90% by volume of total metal powder in flake form and dendrite form.
 6. A conductive film as claimed in claim 1, wherein the metal of said metal powder in flake form is one member selected from the group consisting of copper and silver.
 7. A conductive film as claimed in claim 1, wherein the metAl of said metal powder in dendrite form is one member selected from the group consisting of copper and iron.
 8. A conductive film as claimed in claim 1, wherein the metals of said metal powder in flake form and metal powder in dendrite form are silver and copper, respectively. 